Answers to Mastering Concepts Questions
3.1
1. What is a cell?
A cell is the smallest unit of life.
2. How have microscopes contributed to the study of cells?
Microscopes have led to the discovery of cells, have allowed us to differentiate structures
in prokaryotic and eukaryotic cells, have allowed us to study the organelles that make up
a eukaryotic cell, and have even let us watch some basic cell functions as they occur.
3. What are the main components of cell theory?
The main components of cell theory are that all life is made of cells, that cells are the
fundamental unit of life, and that all cells come from preexisting cells.
4. Describe the differences between light and electron microscopes.
The light microscope can be used to study living organisms, does not distort structures,
can show structures in color, and is less expensive than the electron microscope.
However, the light microscope lacks much of the magnification and resolution of the
electron microscope. Electron microscopes require that structures be killed and can only
produce black and white images.
5. Which molecules and structures occur in all cells?
All cells have DNA, RNA, ribosomes, proteins, a watery cytoplasm, and a cell
membrane.
6. Describe adaptations that increase the ratio of surface area to volume in cells.
Adaptations that increase the surface area to volume ratio include elongated or flattened
cell shapes. An efficient internal transportation system also helps cells sidestep surface
area limitations.
3.2
1. How do prokaryotic cells differ from eukaryotic cells?
Prokaryotic cells lack a nucleus and other membrane-bounded organelles and are
typically much smaller than eukaryotic cells.
2. How are bacteria and archaea similar to and different from each other?
Bacteria and archaea are small cells that lack nuclei and membrane-bounded organelles.
The two groups differ in the composition of the cell wall and the lipids in the cell
membrane.
3. How do organelles contribute to efficiency in eukaryotic cells?
Organelles contribute to efficiency in eukaryotic cells because they concentrate the
biochemicals needed for chemical reactions so that the reactions proceed more rapidly.
Also, because of organelles, a eukaryotic cell can maintain high concentrations of each
biochemical only in certain areas, not throughout the cell.
3.3
1. How does the chemical structure of phospholipids enable them to form a bilayer in
water?
The hydrophilic heads are attracted to water, whereas the hydrophobic tails avoid water.
When many phospholipids join, the hydrophilic heads align facing the water, and the
hydrophobic tails align away from water. The result is a bilayer.
2. Where in the cell do phospholipid bilayers occur?
Phospholipid bilayers form the boundaries of organelles within the cell. They also occur
at the cell’s surface as the plasma membrane.
3. What are some functions of membrane proteins?
Membrane proteins include: transport proteins that move substances into and out of cells;
enzymes that facilitate chemical reactions; recognition proteins that allow the body to
recognize its own cells; adhesion proteins that stick cells together; and receptor proteins
that bind molecules outside the cell and trigger chemical reactions inside the cell.
3.4
1. Which organelles interact to produce and secrete a complex substance such as milk?
The nucleus, ribosomes, rough ER, smooth ER, and Golgi apparatus are organelles
involved in the production and secretion of milk. Mitochondria provide the necessary
energy.
2. What is the function of the nucleus and its contents?
The nucleus contains DNA (the molecule that contains the instructions for making
proteins) and the nucleolus (where ribosomes are assembled). Messenger RNA (mRNA)
molecules are synthesized in the nucleus before they exit to the cytoplasm through
nuclear pores.
3. Which organelles are the cell’s “recycling centers”?
Lysosomes, vacuoles, and peroxisomes are the cell’s recycling centers.
4. What process occurs in a chloroplast?
Chloroplasts are plastids that carry out photosynthesis.
5. Which organelle houses the reactions that extract chemical energy from nutrient
molecules?
Mitochondria house the reactions that extract chemical energy from nutrient molecules.
6. Which three organelles contain DNA?
DNA occurs in the nucleus, chloroplasts, and mitochondria.
3.5
1. What are some functions of the cytoskeleton?
The cytoskeleton provides a structural framework for the cell, is a transportation system
within the cell, allows the cell to move, and connects cells together.
2. What are the main components of the cytoskeleton?
The major components of the cytoskeleton are microfilaments, intermediate filaments,
and microtubules.
3. How are cilia and flagella similar, and how are they different?
In eukaryotes, cilia and flagella are both made from microtubules. They differ in their
length and in how they move. Cilia are short and move in wave-like motions. Flagella are
much longer and move with a whiplike motion.
3.6
1. What functions do cell walls provide?
Cell walls provide rigidity, prevent a cell from bursting if it takes in too much water, and
allow interactions with other cells.
2. What is the chemical composition of a plant cell wall?
Plant cell walls are made of cellulose and other polysaccharides.
3. What are plasmodesmata?
Plasmodesmata are tunnels in the cell walls that connect adjacent plant cells.
4. What are the three types of junctions that link cells in animals?
Tight junctions, anchoring junctions, and gap junctions link cells in animals.
3.7
1. How did the researchers use multiple lines of evidence to answer their question?
The researchers used microscopy with fluorescent tags, altered genes, and studies of
molecules in multiple species as independent lines of evidence in support of their
conclusion that the actin in eukaryotic cytoskeletons has a functional counterpart in
bacteria.
2. How would the bacterial cells in figure 3.25b look different if the target proteins
occurred throughout the cytoplasm?
If the target proteins occurred throughout the cell then the radioactive parts of the cell
would appear throughout and not just at the cell surface.
Answers to Write It Out Questions
1. How did microscopes contribute to the formulation of the cell theory?
The first microscopes allowed early scientists to view cells in cork and to learn that all
organisms are made of cells. As the technology advanced, scientists could see the
internal structures within cells. New microscopes have allowed scientists to view the cells
in much greater detail. Scientists now understand much more about what cells are made
of, how they use energy, and how they divide.
2. List the features that all cells share, then name three structures or activities found in
eukaryotic cells but not in bacteria or archaea.
All cells have DNA, RNA, protein, ribosomes, cytoplasm, and a cell membrane.
Structures or activities unique to eukaryotic cells include a nucleus, mitochondria,
chloroplasts, and the endomembrane system.
3. In what ways is a prokaryotic cell like a baseball stadium, but a eukaryotic cell is more
like an office building?
A prokaryotic cell is more like a baseball stadium because it has a simple structure with
few internal compartments. A eukaryotic cell is more like an office building because it is
more complex and has many different internal compartments and specialized areas.
4. Biologist J. Craig Venter has designed and built an artificial bacterial chromosome. If
he wants to build an entire cell from scratch, what other ingredients will he need? Can
you foresee any benefits from, or ethical problems with, the ability to create artificial
life? What do you need to know to be able to answer these questions?
He will need a membrane in which to enclose the cell, ribosomes, RNA, enzymes, and
cytoplasm. Some benefits could include a better understanding of how life might have
begun on Earth or the ability to manufacture cells with all the desired characteristics.
Ethical questions could arise. Do we have the right to create life? What would be the
risks of introducing completely novel life to current ecosystems? Science cannot answer
the ethical questions, but experiments might help provide information about likely
impacts on ecosystems.
5. Your friend claims that an ostrich egg is the largest single cell, but you are skeptical
that one cell can be that large. If you had access to an ostrich egg and a high-quality light
microscope, what features would you look for to resolve the argument?
One visible clue that the egg is made of just one cell would include the presence of just
one nucleus and just one cell membrane. If the ostrich egg were made of multiple cells, it
would contain multiple nuclei.
6. Suppose you discover a new organism and that you have access to light and electron
microscopes to examine its cells. List a specific question you could answer by using each
type of microscope.
With the light microscope you can determine if the cell is prokaryotic or eukaryotic and
determine whether the cell is from a plant, animal, or other group. With the electron
microscope you can see if the cell contains structures that are too small to see with the
light microscope or that cannot be stained.
7. Why are large organisms made of numerous small cells instead of a few large ones?
Nutrients and wastes must enter and leave each cell through its surface. All cells
therefore require a large surface area through which they can interact with the
environment. If an organism were made of a few large cells, the ratio of surface area to
volume would be too small for each cell to function.
8. Which has a greater ratio of surface area to volume, a hippopotamus or a mouse?
Which animal would lose heat faster in a cold environment and why?
A mouse has a higher surface area to volume ratio and would lose heat more quickly. The
body of the mouse is in greater contact with the cold environment than the hippopotamus.
The mouse also has less volume to generate replacement heat.
9. List the chemicals that make up cell membranes.
The chemicals in cell membranes are phospholipids, sterols, proteins, and sugars.
10. Compare and contrast the phospholipid bilayer with two pieces of Velcro sticking to
each other.
The two pieces of Velcro have the fabric backing sandwiching the hooks and loops
inside, similar to the phospholipid tails sandwiched between the two layers of heads. The
nature of the backing (not able to attach) and the hook and loops (able to attach) are
different, similar to how tails are hydrophobic and heads are hydrophilic. However, what
holds the two pieces of Velcro together are actual attachments between the hooks and
loops; the phospholipids tails do not attach to each other, they just orient themselves
based on their ability to repel water.
11. One way to understand cell function is to compare the parts of a cell to the parts of a
factory. For example, the Golgi apparatus would be analogous to the factory’s shipping
department. How would the other cell parts fit into this analogy?
The nucleus would be the management office where important information is stored and
used, and the rough and smooth endoplasmic reticulum would be the assembly lines for
production. The forklifts would be transport vesicles moving supplies and products
throughout the factory, and the generators would be the mitochondria. The girders and
steel beams that make up the structure of the building would be the cytoskeleton.
12. This chapter used the endomembrane system to illustrate the organelles involved in
milk production. Once a baby drinks the milk, which organelles in the infant’s cells
extract the raw materials and potential energy in the milk to fuel growth?
Peroxisomes break down the lipids in the milk. Enzymes inside and outside of the
mitochondria convert the broken down lipids into chemical energy that the cell can use.
13. Why does a muscle cell contain many mitochondria? Why does a white blood cell
(an immune cell that engulfs bacteria) contain many lysosomes?
Muscle cells require a lot of energy to move parts of the body; mitochondria use aerobic
cellular respiration to extract energy from food. White blood cells have many lysosomes
because they engulf and dispose of debris and bacteria.
14. List the components and functions of the cytoskeleton.
Three components of the cytoskeleton are microfilaments, intermediate filaments, and
microtubules. The cytoskeleton forms a transportation system, provides structural support
necessary to maintain the cell’s characteristic three-dimensional shape, enables cells or
parts of the cells to move, aids in cell division, and helps connect cells to one another.
15. How do plant cells interact with their neighbors through the rigid cell wall?
Plant cells interact with their neighbors via plasmodesmata, which are channels that
connect adjacent cells. Plasmodesmata are essentially tunnels in the cell wall, through
which the cytoplasm and some of the organelles of one plant can interact with those of
another.
16. Describe how animal cells use junctions in different ways.
Tight junctions create a seal between adjacent cells. Anchoring junctions are “spot
welds” that secure cells in place. Gap junctions allow adjacent cells to exchange
cytoplasmic material.
17. List several examples of highly folded organelles with huge surface area.
Examples include the endoplasmic reticulum, Golgi apparatus, mitochondria, and
chloroplasts.
18. Scientists use nanoparticles (tiny objects smaller than 100 nanometers) to deliver
DNA or drugs into animal cells. Are nanoparticles visible with the human eye? With a
light microscope? With an electron microscope? What physical barrier makes it hard to
use nanoparticles to place chemicals inside of plant cells?
Nanoparticles are only visible through an electron microscope. The plant cell wall would
inhibit the use of nanoparticles to insert chemicals inside plant cells.
Answers to Pull It Together Questions
1. What are the functions of each part of a cell?
Plasma membrane – the outer barrier between the cell and its environment. It contains
proteins to aid in movement of materials, cell recognition, enzymatic reactions, and
binding to external molecules and other cells.
Nucleus – contains the DNA and nucleolus; synthesizes RNA from DNA.
Ribosome – synthesizes proteins.
Rough endoplasmic reticulum – synthesizes proteins embedded in membranes.
Smooth endoplasmic reticulum – synthesizes lipids and detoxifies drugs and poisons.
Golgi apparatus – prepares substances to be exported from the cell.
Cytoskeleton – the internal framework of the cell. Supports the cell, aids cellular
division, transports materials, and connects cells to one another.
Mitochondrion – carries out aerobic cellular respiration, which provides energy for
chemical reactions throughout the cell.
Chloroplast – carries out photosynthesis.
Lysosome – breaks down and recycles nutrients and wastes.
Peroxisome – container for enzymes that digest fatty acids and break down toxic
molecules.
Central vacuole – stores substances and regulates volume of plant cell.
Cell wall – stiff structure made of cellulose that provides cell shape in plants.
2. How do a cell’s organelles interact with one another?
Each organelle does only part of the work of the cell: the nucleus stores information,
ribosomes produce proteins, mitochondria produce energy, and so on. Vesicles, or small
bubbles of membrane, carry proteins and other substances from one organelle to another.
Since each organelle has specific functions, vesicles allow organelles to divide tasks
among each other. The production of milk, as described in the chapter, is a good
illustration of how organelles use vesicles to interact.
3. Add the eukaryotic kingdoms of life to this concept map.
All the eukaryotic kingdoms (Protists, Fungi, Plantae, and Animalia) should be listed
under Domain Eukarya with the connecting phrase “contains these kingdoms”.
4. Which structures occur in plant cells but not animal cells?
Plant cells have chloroplasts, a central vacuole, and a cell wall.
5. How do plant and animals cells stick together and communicate with their neighbors?
Plant cells stick together at their cell walls. Adjoining cell walls share gaps called
plasmodesmata, which allow the free flow of nutrients and chemical signals between the
cells.
Many animal cells are connected by complex protein structures called junctions.
Anchoring junctions tightly link the intermediate filaments of neighboring cells. Large
pores, called gap junctions, connect other cells. Tight junctions hold cells completely flat
against each other, with proteins extending from one plasma membrane into the other.
Communication is possible between some of these connections. Cells sharing a tight
junction can readily share chemical signals with each other by exporting them through
their membranes. Neighboring cells held in place by anchoring junctions can likewise
send chemical signals to each other, but not as efficiently. Gap junctions allow cells to act
in unison by allowing the free flow of nutrients and chemical signals.
6. Which cell types have a cell wall?
Plants, algae, fungi, bacteria and archaea all have cell walls, but the walls are made of
different materials.